LR 4 is experiencing some performance "issues" even on better speck machines, it might pay to hang on till you've tested it properly with a good number of big images.
It also may not show LR3 presets properly due to the tone curve differences, and the new slider set up.
Adobe also advise you don't update all your images to process 2012 in one go, but do them one by one due to the different rendering.